Laurier's Handy Dandy Little Circuits Digital Capacitance Meter by wls73800

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									Laurier's Handy Dandy Little Circuits: Digital Capacitance Meter




                             Introduction     construction-1       Construction-2   Construction-3   Calibration


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     Note
     The Digital Capacitance Meter ( DCM ) circuit and PCB was designed by Laurier Gendron. It is
     being made available to hobbyists for personal development only. It cannot be used for
     commercial purposes of any kind without previous written permission. ( 6 Oct. 2000 )

     Introduction

     While I was looking over some old electronic publications and I came across a design for a
     two digits capacitor meter using 555 i/c timers and my mind immediately flashed to a three digit
     counter design published in the Motorola data manual using their three digit decoder driver chip
     MC14553 .
     As I studied the design I decided to explore the possibility of using simple available
     components to measure capacitance and came up with a simple design within reach of anyone
     at a fraction of the cost of a commercial unit.
     You can make a search and download application data sheets for all the ICs used in this
     project from ON semiconductors

     Circuit Description
     As a reference you may wish to open up a new window to view the circuit while reading the description. Resize
     as required .

     The three digits counter is made of CMOS ICs MC14553 and MC or CD14543 , three common
     cathode digital displays , three PNP transistors and current limiting resistors .

        The Counter Display __ We need to know how the counter display operates in order to
     make use of it , the heart of the counter is the MC14553 which is a three-digit BCD counter and
     with the use of the MC14543 a BCD-to-seven segment decoder / driver will decode and
     activate the proper digit segments to display a maximum display count of 999.
     To do this the MC14553 needs three input signals, a positive (high) pulse to the Latch input (
     pin #10 ) to enable storage of pulses to be stored into the latch and a Reset ( pin # 13 ) pulse (
     high ) to reset the counter . The total count desired is controlled by the Latch action which sets
     the time we require to insert the amount of pulses to the Counter ( pin #12) to be displayed.




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     When the latch is high the count starts and when the Latch is low (zero) the count is stopped
     and the total of pulses accumulated in the latch are displayed then a positive pulse is required
     to the Reset to clear the Latch , the speed of this process is controlled by an internal 100kHz
     oscillator which is determined by the .001 capacitor connected between pin # 3 and pin # 4 .

          The Input Logic Circuit

        In addition to the three digits counter circuit for display , the measurement of a capacitor can
     be achieved with only two additional CMOS ICs ,MC or CD4011 and MC or CD4049 , a rotary
     switch and a few resistors and capacitors.




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        The Oscillator__ Now that we know what controls the counter we use the Oscillator to
     measure the value of the unknown capacitor .
     The oscillator is made of two of the six independent buffers of the MC4049 which is an HEX
     Inverting Buffer . Our test capacitor is connected between pin # 3 and # 4 and with the
     selection of one of the chosen resistors connected between pins # 2-6 and # 3 we have
     oscillation . The scaling fixed 1% resistors (10 meg , 1 meg ,100K , 1K ) were selected to
     produce a reading of " 010 " on all ranges except picofarad range .( see table )
     The resulting square wave frequency from the test capacitor at pin # 4 is passed on to the next
     two buffers which are used as a current amplifier, the output of pin # 15 is then connected to
     pin # 13 of the first gate of MC4011 the action of which we will discuss in the pulse shaper
     section.

         The Counter__ In the mean time the counter is always active and oscillating and its
     frequency is selected by the switching arrangement to coordinate with the range selected for
     the capacitor under test .
     Exactly the same circuit arrangement is used except that the resistance in use could be one
     fixed resistance if we had capacitors of 1% accuracy but their price prohibit such luxury so we
     have to compensate by using adjustable resistances to calibrate the frequency to be fed to the
     counter for a reading that reflects the capacitor value under test .
     The counter output is connected to pin # 12 of the first gate of MC4011 , it is a Quad NAND
     Gate . Both input must be high to get a low output , two low inputs give a high output and a low
     and high input gives us a high output. These conditions are used to form the Latch and Reset
     pulses that control the three-digit counter .

          The Pulse Shaper__ As mentioned earlier MC4011 is used to shape the output of the


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     oscillator pulses , from pin # 15 of the MC4049 capacitor C5 is used to couple and filter the
     square waves into triangle waves to one gate of MC4011 at pins # 1-2 , C6 is used to pass on
     the inverted pulses to pins # 5-6 of the following gate . Resistors R4 and R5 are used to pull
     down the square waves to sharp pulses used for the Latch and Reset .
     As the oscillator counter puts out a constant stream of positive square waves to Pin # 12 of the
     4011 the input at pin # 13 from the amplifier automatically interrupt the output count at pin # 11
     allowing for the right number of pulses when correctly calibrated to be registered in the Latch .

     Functions
        The meter will measure capacitance from a low 10 picofarad (or lower if the wind is right ) to
     10,000 uF in five ranges as follows ;
                        Select      Range                                                      Test          Display
                        Pico        1 pico to 999 picofarad (extended range )                  *10 pico 010
                        .001        .001 to .099 uF (microfarad )                              .01           010
                        .1          .1 to 9.99 uF ( Next show x10 when used )                  .1            010
                                    1.uF to 99.9 uF ( x10 of previous range ,
                        1.uF                                                                   1.uF          010
                                    optional )
                        10 uF       1.uF to 99.9 uF                                            10 uF         010
                                    100 uF to 9999 uF ( Optional with x10 switch
                        100 uF                                                                 100 uF        010
                                    )



                               Introduction   construction-1       Construction-2   Construction-3    Calibration




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                                    Introduction   Construction-1   construction-2   construction-3   Calibration


                           As a reference you may wish to open up a new window to view the circuit while checking the list.

       Parts List

          RESISTANCES (all 1/4 W )
          ¤ R1              10 meg 1% ( or measured )
          ¤ R2              1 meg 1%         . .
          ¤ R3              100K   1%        . .
          ¤ R4              1K     1%        . .
          ¤ R5 R6           10K to 47K (see text )   10%
          ¤ R7 to R13       390 ohms                 10%
          ¤ R14 to R16      1K                       10%
          ¤ RV1 to RV3      7K ( total resistance , see text )

          CAPACITORS
          ¤ C1                         .001 uF 5%
          ¤ C2                         .01 uF 5%
          ¤ C3                         .1 uF   5%
          ¤ C4                         1.uF Tantalum
          ¤ C5,C6,C7                   .001 uF Polyester
          Filter capacitors            .1 Poly. ( ICs by-pass )
          Filter Capacitor             200 uF/15V (supply )

          ICs
          ¤ IC1          CMOS, MC14553 , 3-Digit BCD Counter
          ¤ IC2          CMOS, MC or CD 14543 , BCD-to-7-Segment Latch/Decoder/Driver
          ¤ IC3          CMOS, MC or CD 4049 , HEX Inverting Buffer
          ¤ IC4          CMOS, MC or CD 4011 , Quad 2-Input NAND Gate .
          ¤ IC4        * A 4093 Schmitt trigger can be used and is direct substitute for the 4011


          Transistors
          ¤ Q1 to Q3 2N2907 or similar PNP


          Others
          ¤ Sockets   16 pins,3 ea
                      14 pins,1 ea
          ¤ Display   3 ea Common Anode LED display of your choice
          ¤ Switches 1 ea ,3- poles 4-positions Rotary switch
                      1 ea ,1-pole 1-position miniature toggle or slide switch ( for ON/OFF )
          For Benchwork model
          ¤ Switch knob , case, Display red filter, Capacitor test socket or short lead wire
          with clips , battery holder .


          Optional
          ¤ VR               Voltage Regulator 780x (1.5 A                        TO220 ) positive voltage as required
          (optional)
          ¤ Bridge           Rectifier bridge 50V/1A ( optional)
          ¤ Switch           One small normally open push button switch                                   ( Supply ON/OFF and Test )

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          For use with battery supply
          ¤ Socket    14 pin dip for 7 display current resistors

           Construction
               Notes
           This project may be not suitable for the beginners as a certain amount of experience is required to
           achieve a good operating system , nevertheless the following construction suggestions should help
           everyone in preventing unwanted problems .
             Unless you never make mistakes use sockets for all the ICs.
             Keep all leads especially capacitor leads as short a physically possible.
             Do not use ceramic capacitors as they are temperature sensitive and will be unstable and produce
           noise.
             Best operating voltage is between 9 volts and 12 volts. Operation down to 6 volts is possible but with
           loss of sensitivity in the picofarad range. Display current limiting resistors values R7 to R13 will need to
           be adjusted to control brightness level below 10mA per segment . I used a 14 pins IC socket for R7 to
           R13 for quick adjustments . No need to mess up the PC board .
             If and external power supply is used AC or DC install a regulator as well as a large filtering capacitor
           (200 uF) for stability.
             If you decide to use a battery pack as your source of supply consider using a normally open push
           button switch in the supply line to prolong batteries life.
             The rotary switch must be installed as close as possible to the capacitors , excessive long leads will
           result in a noisy system and unstable display.
             The following will produce noise ; bad capacitors , poor wiring, cold solder , twisted wiring , bad
           connections , noisy supply lines , dirty switch contacts .
                                    Introduction   Construction-1   construction-2   construction-3   Calibration




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                                Introduction    construction-1     Construction-2   construction-3   Calibration



            As a reference you may wish to open up a new window to view the circuit while on this page .

                I have designed two models to choose from . One which I will call the WORKBENCH
            and the other the PORTABLE .
            The basic design is exactly the same in both but the layout is different as shown . I have
            listed the differences in their appropriate window.

                                  Work
                                  Bench
                                  Model is
                                  designed to
                                  be used
                                  with a side
                                  vertical
                                  mounted
                                  switch to
                                  allow very
                                  short leads
                                  to
                                  capacitors
                                  and other
                                  components
                                  to minimize
                                  noise and
                                  stray
                                  oscillation.
                                  It also
                                  provide as
                                  an option
                                  PC layout
                                  for a
                                  regulated
                                  supply
                                  source.




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                                                                   On the left
                                                                   is the PCB
                                                                   tracing .
                                                                   On the right
                                                                   is the PC
                                                                   Tracing
                                                                   with
                                                                   components
                                                                   overlay and
                                                                   suggested
                                                                   switch
                                                                   mounting .




               In both models the red tracing is on the components side and when using the PCB or
            the point to point wiring it is easy to install jumpers made of # 26-30 size bare wire under
            the sockets before installing the sockets then solder the pins in place. Once the jumpers
            under the sockets are installed , all that remains to wire on the components side is Positive
            supply bus which should be done once all the sockets are installed .

                                  The
                                  Portable
                                  model
                                  shown on
                                  the right
                                  was
                                  designed so
                                  that the
                                  switch can
                                  be mounted
                                  directly on
                                  the PCB or
                                  hand wired
                                  board .
                                  The switch
                                  is shown
                                  mounted on
                                  the
                                  components
                                  side .
                                  A low
                                  battery
                                  voltage
                                  monitor
                                  circuit was
                                  added .


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                                                                     The switch
                                                                     on the
                                                                     Portable
                                                                     model can
                                                                     be
                                                                     installed
                                                                     on the foil
                                                                     side BUT
                                                                     remember
                                                                     to reverse
                                                                     the order
                                                                     of the
                                                                     switched
                                                                     component
                                                                     .




              Both of my prototypes were point to point wired on perforated board using #26 bare wire
            duplicating exactly the PCB layout . Naturally I always use sockects .
            Once this is done it is a simple matter to use a Ohmeter and check all wiring connections
            on the sockets pins , any error will be readily detected . I then apply power and check for
            any current that would indicate a short ,remove power ,insert the IC's one by one ,apply
            and remove power after each IC's while monitoring for excessive current .

              Both circuit models can be easily secured to the wall or cover of the enclosure by the
            switch mounting screw nut .

               For the Workbench circuit , I used a 12 volts plug-in supply for the work bench model
            through the on-board installed 9 volt regulator and used an ON/OFF miniature toggle
            switch and added a push button for the X10 range.

               A six "AA" battery pack is used for supply to the Portable circuit and a normally open
            push button switch is used to turn it on and make measurements. An LED was installed on
            the case and wired to the Battery Monitor circuit . A miniature toggle switch is used for the
            10x range.
                                Introduction    construction-1     Construction-2   construction-3   Calibration




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                            Introduction       construction-1   Construction-2   Construction-3   Calibration



              As a reference you may wish to open up a new window to view the circuit while on this page .

              Technical Info
                 LED displays of all sizes and brightness are available , a best buy is to get a
              three to four digit stick that is already multiplexed . For those who are not familiar
              with their use I include some details and drawings for your information . Use
              lengths of about 3" of rainbow coloured #24 wires for easy identification as a
              harness and solder each end to the circuit board . Make sure you identify
              correctly which wire goes where .




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                 Switches
              Details of the Rotary switch wiring is shown below . As an option two more
              ranges will be added when a SPST switch is used to parallel a 1.uF capacitor to
              the .1uF of range 3 &4 . If you install the rotary switch on the foil side don't forget
              to reverse the order of the resistances and capacitors on the switch . Also shown
              is the dimension of the switch to fit the PCB .




                 Circuit diagrams for the Low Voltage Monitor and the Voltage Regulator are
              shown. A suitable voltage regulator of your choice can be used , a TO220 1.5 A
              regulator must be used as the current will reach up to 130 mA in certain
              conditions , no heat sink is required .

                 The Low Voltage Battery Monitor circuit shown is calibrated for a 9 volts
              supply , as it reaches a low voltage of 7 volts the LED will come on indicating


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              need for batteries change .
              R1 and R2 form the calibrated set point voltage divider to pin # 3 of the 741 and
              can be replaced by a 10K variable resistance to calibrate for other voltages
              above 6 volts.




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                        Introduction      construction-1      Construction-2   Construction-3   Calibration



               As a reference you may wish to open up a new window to view the circuit while on this page
               .

               Calibration
                    Optional Ranges . By adding the 1.uF capacitor in parallel to the
               .1uF capacitor we obtain a x10 expansion of range #3 and # 4 . I did not
               want to use a six position switch when a simple SPST switch can do the
               work only when very large capacitors need to be tested . On the other
               hand some may object to having both ranges 3 & 4 expanded when
               only expansion of the last range is needed .
               If this is the case , removed the jumper between pin # b-4 and # b-5 and
               install an additional .1 uF Capacitor from # b-5 to pin # 12 of IC-4049
               thus the range # 4 will not be expanded . You can mount the additional
               capacitor directly on the switch lug .
               I was very pleased to be able to measure capacitance as small as 3
               pico with nary a flicker of the display and I hope you can get the same
               results.

                 Before you start you calibration ensure that the supply voltage is
               exactly the one you will be using for the system operation then proceed.
               Calibration is easy and can be accomplished two ways;
               1 - If you have a frequency meter adjust RV1 to RV3 for each range to
               the approximate following frequency ( reading from pin # 12 of IC-4049 )
               . I say " approximate " because IC's like other components are not all
               exactly the same . Of the three MC 4049 I used for testing I found a
               range of 5 kHz difference , so it is not exactly a matter of timing but
               rather matching two oscillators.
                                                         Picofarad 85 kHz
                                                         .001         8.5 kHz
                                                         .1           850 Hz

               1 - If a frequency meter is not available the best result will be obtained

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               by measuring known value capacitors that will read in the lower three
               digits range as follow:
                                                RANGE TEST VALUE DISPLAY
                                                Pico       100 pico   100
                                                .001       .01        100
                                                .1         1.uF       100
                                                10. uF     100 uF     100


               Final Notes
                  For simplicity I used two short lengths( 4 ") # 22 stranded lead wires
               with a small hook type connector at each end . With the test leads open
               ( no test capacitor connected ) the first digit display should read zero but
               a small residual reading of 1 or 2 may be seen and this residual should
               be added to capacitance reading below 100 pico.These conditions are
               normal and are the product of the very high internal oscillation with open
               leads when in the Picofarad range .The next two ranges should read
               zero on the first digit and all zero on the Microfarad range .

                  Range 4 and 5 do not need to be calibrated, but if you wish to do so ,
               add and additional RV to switch # a-4 and remove the jumper between
               pins #a-3 and # a-4 , the other end connects to pin # 9 of IC4049 .

                  The total recommended RV's resistance is 7K ohms , a combination
               of one resistance named " R " on the layout is used in series with the
               RV's so that any RV of a smaller value will make it easier to calibrate
               the required frequency . If " R " is not used merely install a jumper
               across the PC connection points .
               The actual total resistance setting for RV's on my prototypes was
               measured as 6K ohms so a combination of 5 K for " R " is series with 2K
               Rv's would make for easier calibration . The other approach is to use 10
               turns trimmers of 10 K ohms .

                 It is greatly satisfying to be able to reach a set goal developed from
               an idea and I am well pleased with the outcome , now the owner of a
               Digital Capacitance Meter that I can trust and rivals professional units .
               Have fun and tell me how yours turns out and I will be glad to answer
               any of your queries.

               Where
                    For reference only , the following products descriptions were found

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               listed in the Digikey catalog , prices are in Can-$ .
                                                                                           Found one exactly the
        Rotary                                                        Cat #                same" Lorlin-UK"
                           3-poles,4-positions ,single                           (Can. ) $
        Switch PC                                                     EG                   from Intek Electronics,
                           deck                                                  7.92
        lugs                                                          1955                 Vancouver,for
                                                                                           Can$2.95

                      3-digits .5" H ,red/orange/green (                        Cat # 67-1423 ND ,
       Displays                                                                                               $ 7.95
                      multiplexed )                                             common anode
                      ( Panasonic ) .3"H Cat # P232-ND,
                                                                                ( Can ) $ 2.44 , 2.45,
       Single Display .43"H Cat # P326-ND , .57"H Cat #
                                                                                2.53
                      P326-ND ( common anode )
                                                                                                              $ 1.98
                                                                                From Newark
       MC 14553              Cat # 07F4166                                                                    U.S , Can
                                                                                Electronics
                                                                                                              $ ??

      Newark Electronics
      U.S.A
      Canada
                        Introduction      construction-1      Construction-2   Construction-3   Calibration




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